Processes for the production of stable, aqueous polyurethane-polyurea dispersions are already known and are described for example in German Federal Pat. Nos. 1,184,946 and 1,178,586; German Auslegeschrift No. 1,237,306; German Offenlegungsschrifts Nos. 1,495,745; 1,595,602; 1,770,068; 2,019,324 and 2,314,512 and U.S. Pat. Nos. 3,388,087; 3,480,592; 3,461,103 and 3,686,108; see also D. Dieterich et al., Angew. Chem. 82, 53 (1970). The dispersions described in the literature mentioned above are prepared on the principle of incorporating hydrophilic centers into a macromolecular chain of a polyurethane (polyurea) molecule. In the known dispersions, these hydrophilic centers or so-called internal emulsifiers are ionic groups or ether functions. The groups are either incorporated in the prepolymer in the form of special diols or used as modified amines for chain lengthening the prepolymers which have each at least two isocyanate functions in end positions.
Various processes for preparing the known dispersions have been described, for example in D. Dieterich and H. Reiff, Angew. makromol. Chemie 26, 85 (1972). The method employed generally consists of either converting the solution of a polyurethane in an organic solvent into an aqueous dispersion or of dispersing a liquid prepolymer stage with or without solvent in water. For example, a liquid prepolymer ionomer containing isocyanate groups may be introduced into water with vigorous stirring to form an emulsion of the prepolymer which then continues to react with water or a diamine or polyamine to undergo chain lengthening to a high molecular weight polyurethane urea.
One method of dispersion which is particularly simple to carry out has been described in German Offenlegungsschrift No. 1,913,271 and/or U.S. Pat. No. 3,756,992. According to this method, a solution or liquid polyurethane polyelectrolyte which is capable of undergoing an addition reaction with formaldehyde to form methylol groups is dispersed by mixing it with water and is converted into a polyurethane polyelectrolyte containing methylol groups by the addition of formaldehyde or formaldehyde derivatives. This polyurethane polyelectrolyte is then condensed either in the dispersion or on a substrate to form high molecular weight polyurethane.
The dispersion is prepared by adding water to the stirrable melt until the water forms the continuous phase, the mixture generally first passing through the stage of a water-in-oil emulsion. Alternatively, a water-in-oil emulsion may be prepared at elevated temperature and left to cool, until it changes into an oil-in-water emulsion. The preparation of the dispersion is almost always carried out at elevated temperature, preferably at between 50.degree. C and 120.degree. C. This is necessary, first, because the viscosity of the prepolymer stage which is to be dispersed is too high at room temperature so that dispersion at room temperature would require elaborate apparatus such as screw extruders, and secondly, because dispersion generally proceeds more rapidly with increasing temperature. The dispersion of solvent-free melts of isocyanate prepolymers is also nearly always carried out at elevated temperature.
Although the method described above is economically very advantageous compared with the preparation of dispersions from solutions and can be achieved with very simple means, it does not meet all of the practical requirements. For example, nonionic dispersions are impossible or at least very difficult to prepare by this method because the dispersibility of polyurethane prepolymers containing hydrophilic polyether groups decreases with increasing temperature. This means that if dispersion is to be carried out at the necessary elevated temperatures, the polyurethane prepolymers must be rendered more hydrophilic than is necessary for the subsequent stability of the dispersion. The resistance of the product to water is thereby adversely affected. On the other hand, the preparation of polyurethanes modified by hydrophilic ether groups would be desirable because such dispersions have a high resistance to frost.
The usual ionomer dispersions are generally insufficiently frost resistant so that their storage and transport in cold weather are expensive because they require heated storage rooms. In addition, the transport of aqueous dispersions over long distances is in any case unsatisfactory because the transport of large quantities of water is inevitably expensive.
There is, therefore, an urgent need for the manufacturers using the product to be able to prepare their own dispersions from the solid materials and water in much the same way as they normally prepare their own organic solutions.
The known redispersible powders are a step in this direction. The manufacture of these powders is technically complicated and economically not of advantage because the powders are produced from a ready-made dispersion which has to be converted into powder by a cost intensive process of freeze drying or spray drying as described in German Auslegeschrift No. 1,729,201. It would, therefore, be desirable to have a process for the production of a solid material which is stable in storage and could be converted to an aqueous dispersion at a later date without having to pass through the complicated aqueous dispersion stage described above.
The full economic advantage of using polymers from the aqueous phase could then be obtained.
The solution to this problem has in the past generally been regarded as fundamentally impossible because dispersions, being metastable diphasic systems, cannot spontaneously be produced by a solution process but, on the contrary, tend to separate irreversible solids under the influence of chemical or physical changes, and for this reason the stabilization of dispersions is commercially of great importance. It is in fact true, as is well known to any expert in the field, that coatings deposited from dispersions are generally impossible to reconvert into a dispersion by the action of water. The commercial applications of dispersions are based on this impossibility of simple redispersion. Exceptions are highly hydrophilic dispersion coatings, but these have no commercial importance on account of their insufficient resistance to water.
It has now surprisingly been found that certain solid prepolymer stages of polyurethane dispersions are spontaneously converted into stable dispersions by contact with water at temperatures below their softening range so that lumps or coarse powders of such solid prepolymer stages can be so to speak "dissolved" or converted to dispersions by introducing them into water or pouring water over them. Dispersions of such prepolymers nevertheless form completely water resistant, high quality films and coatings when chain lengthened with cross-linking agents.